Photochromism is a well-known physical phenomenon, which is observed with certain classes of compounds. Under irradiation of polychromatic light or monochromatic light, photochromic dyes will turn from colorless into colored or from one color into another color. When the irradiation is removed, or under different irradiation from the first polychromatic light or monochromatic light, photochromic dyes will change back to their original colors. A detailed description of this phenomenon can be found in “Photochromism: Molecules and systems”, Studies in Organic Chemistry 40, Eds. H. Durr and H. Bouas-Laurent, Elsevier, 1990.
It is well known that 2H-naphtho[1,2-b]pyran compounds are capable of exerting a photochromic effect. For example, U.S. Pat. No. 4,826,977 describes a series of yellow/orange coloring 2H-naphtho[1,2-b]pyrans containing a spiro-adamantane group at the 2-position, and isomeric naphthopyran systems. In addition, a range of purple/blue coloring 2-(4-aminophenyl)-2-alkyl-2H-naphtho[1,2-b]pyrans have also been disclosed in U.S. Pat. No. 4,818,096.
A disadvantage of the present naphthopyran compounds lies in that their wavelength for the photochromic effect is not long enough, which means that the resulting color is not the ideal green or bluish green. The currently available naphthopyran on the market with the longest wavelength is the product of company JAMES ROBINSON with the trade name REVERSACOL GRAPHITE. However, the first absorption peak λmax of said product in toluene is 486 nm, and the second one λmax in toluene is 593 nm, wherein only the first λmax plays a major role.
As described above, an ideal commercial photochromic lens is supposed to take on a color of grey with a bit bluish green in the sun. The common solution to this problem is to use naphthopyrans in combination with naphthoxazines. An example of said naphthoxazines is the product by JAMES ROBINSON with the trade name REVRSACOL SEA GREEN. But the naphthoxazine compound has a rather poor heat resistance: when temperature is higher than 40° C., its photochromic effect decreases drastically and cannot be used in combination with the naphthopyran compound.
US 2005/0254003 discloses a process for preparing a photochromic polymeric composition. The invention provides polymers having photochromic property and being capable of filtering at least a portion of blue light incident thereon within sufficient mechanical properties.
Therefore, people attempt to solve the aforesaid technical problem, that is, to make the absorption wavelength of the naphthopyran photochromic compound longer, and moreover to retain a good heat resistance. A traditional method to this end is to change the substituent(s) of the naphthopyran compound. However, the resultant product cannot ensure that the effect of absorption wavelength red shift could be achieved as expected after mixing the dyes with the polymer matrix to form articles.